Is it possible for me to destroy multiple segments of a wall if I have multiple targets? Also, could I target a segment of wall and an opposing character with the same attack?

Yes to the first, as long as you have enough damage as well.

No to the second, because there's a line in the rulebook that specifically prohibits that.

To note, you need 3 damage for each piece of terrain you are destroying. so 6 damage for two targets, and 9 for 3 targets. Just rememeber you can't target a character and an object/terrain.